USA (8 Feb 2005) -- Diving mammals are able to propel themselves through the aquatic environment very efficiently. Humans can become good swimmers, but human swimming feats can hardly be compared with those of diving mammals. Techniques for surface swimming, without diving gear, and for breath-hold diving among humans are quite different than those of the mammalian breath-hold divers. Movement through water requires a propulsion system and energy to drive the system. The mechanics of moving a body through water is achieved through the musculoskeletal system, with the skeleton providing the structural support and lever arms for muscle movement. The skeletal system is divided into two main sections, axial and appendicular. The axial skeleton includes the bones in the center of the body, namely the spine, rib cage, and pelvis. The appendicular skeleton includes the bones that support the extremities. The anatomy of the axial and appendicular skeletal systems of diving mammals has advantages for movement as well as for heat and energy conservation in the aquatic environment. Joints of the axial skeleton While bones provide support, joints make it possible for movement between the rigid bony segments to occur. The bony structures-primarily vertebrae-in the axial skeletons of diving mammals and humans are similar. The vertebrae are rectangular-shaped bones with archlike structures on their back edges that provide a canal through which the spinal cord passes. The ends of adjacent vertebrae, the articulating surfaces, are nearly flat. Round pads of fibrous tissue with jelly-like cores, termed intervertebral disks, separate one vertebra from another and act as shock absorbers. The net effect of this anatomy is that only a few degrees of flexion and extension or twisting and bending are possible between two adjacent vertebrae. However, when these 5 degrees to 10 degrees of motion are multiplied by 24 vertebrae in the human-and by even more in porpoises and whales if the tail segments are included-a substantial amount of motion is possible. This mobility allows almost a 180 degrees up and down arc of motion in these diving mammals' flukes. The structural differences between the fins of diving mammals and the extremities of humans eliminate the need for appendicular muscles in the animals' fore fins, flippers, and flukes. The chief method of propulsion for seals and their relatives is through the fore flippers, while in whales and porpoises it is through their flukes. The muscles that move the fins and flippers mostly lie in the axial skeleton and are connected by tendons to the appendages. The shoulder joints of humans and diving mammals have similar bony components (see figure). Functionally, however, they are quite different. The shoulder joints of humans allow tremendous mobility, with the arms having 360 degrees arcs of motion in two planes (front to back and side to side). The shoulder motions of the diving mammals occur in more of a backward-forward direction to move and steer them through the water. Swimming versus diving Muscle activity needed for fast swimming on the surface is quite different from that needed for breath-hold diving. There are indications that diving mammals minimize swimming movements to increase the durations of their breath-hold dives. Just before starting their descents, they exhale fully. The resulting change in buoyancy may be sufficient to allow these animals to descend passively, minimize swimming movements, and thereby conserve oxygen. Even if their buoyancy is only neutralized by the exhalation, which may be the more desirable choice, momentum gained by a few downward swimming movements may be sufficient to allow them to continue their downward descent with minimal energy costs. Correspondingly, when they are ready to ascend, a few upward swimming movements could initiate the momentum to sustain ascent. This would minimize energy expenditures and reduce the chances of depleting oxygen stores and blacking out during ascent. Buoyancy control and energy conservation techniques are utilized by human breath-hold divers (Japanese ama) also. The shallow-water Japanese cachido (or unassisted) ama swim to depths of 15 feet of salt water (FSW) (4.6 meters of salt water) to harvest their foodstuffs on the bottom and then swim back to the surface.1 Bottom times average 30 seconds; total dive times, one minute; and surface interval rest and recovery periods, one minute. Their deep-diving counterparts (funado, or partially assisted divers) rapidly descend passively with weights to 60 FSW (18 MSW), spend 30 seconds swimming on the bottom harvesting food products, and then are pulled to the surface by their assistants. Descents and ascents take 15 seconds each so that the total underwater time is about a minute. The funado divers' surface intervals are 60 seconds. Consequently, the energy expenditures of the shallow-water Japanese ama are greater than those of their deeper-diving counterparts even though the two groups spend approximately equal amounts of time submerged and at rest. These divers through experience have developed the optimal diving patterns for each of their dive profiles.2 Human swimming feats can hardly be compared with those of diving mammals. World-class competitive swimmers can sprint for brief intervals, from 50 to 200 yards or meters, at approximately 4.5 mph (7.2 kph) and for long distances at 3.5 mph (5.6 kph). The range of these swimming speeds is from 22% to 28% of porpoises' speeds. Analyses of upper and lower extremity propulsive efforts in human swimmers show differences between the energy expenditures and efficiencies of these paired appendages. The energy expenditures of the kick, which corresponds to some extent to the fluke movements of porpoises and whales, are two to four times greater than those for the arm stroke, which corresponds to the fore-flipper movements of seals.3 Research has shown that efficiency of the leg strokes varies from 0.05% to 1.23% while that of the arms varies from 0.56% to 6.92%, demonstrating that arm strokes are five- to 10-fold more efficient than the kick. Oxygen consumptions were four times as great for the legs as for the arms in 15-yard (14-meter) swims at 1 yard per second. The differences in efficiency and oxygen consumption between the arms and the legs have two explanations: first, the propelling movements of the legs are relatively inefficient when compared to those of the arms with their greater mobility; second, the muscles of the hips and lower extremities are among the largest in the body and correspondingly have the highest oxygen demands. Long-distance swimmers apply these principles to their swimming by emphasizing the arm strokes while reducing kicking to slow, efficient movements to maintain stability. The most efficient swimming rates for underwater swims with fins are 0.7 to 0.9 mph (1.1 to 1.5 kph), or about 5% of the maximum swimming speeds of porpoises.4 At greater speeds, efficiencies decline progressively based on oxygen consumption rates. Marked variations are observed with different levels of experience, training, body builds, and water temperatures.5 Swimmers with the lower kick rates and the most nearly neutral buoyancies tend to have the highest swimming efficiencies.6 Buoyancy control and energy expenditure are inversely related. One of the most frustrating experiences for human scuba divers is the attempt to maintain a constant depth when too positively buoyant. Swimming in the head-down, feet-up position distracts from the dive and rapidly depletes the scuba air supply. Other techniques to reduce turbulence and drag In diving mammals, subcutaneous fat aids in reducing drag as well as protecting the animal from cold water. The subcutaneous fatty tissue is of an oily consistency. Its pliability conforms to water turbulence patterns and thereby further reduces drag as the diving mammals move rapidly through the water. This adaptation is not found in the competitive swimmer. However, to improve their swimming speeds, swimmers wear swim caps and shave their body hair to reduce drag on their bodies while swimming at top speeds. | | Thin neoprene wet suits frequently used by open-water swimmers not only offer thermal protection, but may also improve performance by reducing drag and increasing buoyancy. Yet maximum swimming speeds of world-class swimmers are only about one-fifth those of the porpoise. Consequently, turbulence and drag effects are much less of an impediment to fast swimming in the swimming human than they are in the mammalian diver. Technology to improve aquatic propulsion Swimming fins are used to improve the propulsive ability of human divers in the water. Fins increase the efficiency of moving through the water by 2% to 8%.3 This level of efficiency corresponds approximately to that of the arm stroke. Yet swimming with fins is only 20% as efficient as walking or running on dry land.7 Swim fins increase the surface area of the lower extremities. However, there is a limit to the effectiveness to be gained from increasing surface area. This is because of the power required to move the increased surface area and the decreased mobility that results with increasing fin size. It is interesting to note that the surface area of the porpoise's tail fluke represents less than 5% of its total body skin surface area. Consequently, size is not the only factor to be considered when evaluating the effectiveness of swim fins. Design, comfort, and mobility are important considerations in deciding which swim fins to use. In the past, progressively larger and more rigid swim fins became popular. These had names such as Jet Fins and Rocket Fins. However, unless the diver had an exceptionally powerful leg kick, they were hard to use because operating them took so much energy. In terms of length, swimming fins vary from short-less than 8-inch (20-cm) projections from the ends of the toes-to more than 3 feet (1 meter) in length. The longer the fins, the more flexible they tend to be. This latter fin choice is very popular for the breath-hold diver who does spearfishing. The long fins are used presumably for their energy-conserving potential during the dive. A few kicks initiate forward movement, and momentum then continues the diver's progression. This can conserve energy during descent and ascent, especially if the diver is neutrally buoyant. At the other end of the swimming fin spectrum is the Zoomer fin used by competitive (surface) swimmers for lower extremity training purposes. The fin is short, rigid, and designed for continuous kicking movements. Speed is increased by about 20% with the Zoomer fins, given comparable kicking efforts, in relation to speed without the fins. For scuba divers donning gear such as tanks, regulators with dive computers, and other monitoring gauges, weight belts, buoyancy compensators, knives, spearguns, game-holding bags, underwater photography equipment, and underwater lights, a fin intermediate in length between the two extremes is most popular. Intermediate-length fins are long enough to provide sufficient forward thrust to move the diver through the water, yet they provide enough mobility to allow the diver to constantly change direction and depth. As with other types of diving equipment, manufacturers have provided many options in fin design for the sport diver. There are options that improve the fins' propulsive ability. Semirigid spines, along the edges and sometimes in the central portions of the fin blades, add stability while still allowing the majority of the blade to be thin and flexible. A reported disadvantage of this design is that the fins are "too responsive" to foot movements. This makes it difficult for the diver to maintain position in order to stalk marine creatures or take underwater photographs. Channels and grooves incorporated into the blade or at the junction of the foot portion and the blade, or both, are supposed to decrease turbulence at the critical points of the fin, which are at the junction of the rigid foot portion and the mobile blade portion. Force fins, with relatively short curved blades and extensions along the blade edges, are purported to increase thrust through a water-scooping effect during the upward portion of the kick. Similar benefits are ascribed to fins with hinge devices or interruptions in their spines at the fin's critical foot-blade junction. The hinges are supposed to help maintain thrust at the endpoints of the upward and downward movements of the kick while reducing turbulence associated with the fin's change of direction. Split fins are another option. The fin blade is split in two. This tends to cause vortices between the splits that aid in forward thrust and may prevent churning up of silt when the diver is swimming near the bottom. Another consideration in fin selection is the type of foot portion. The pocket type with a heel strap is the most popular today. Usually the pocket is large enough to allow the diver to wear neoprene booties with protective rubber soles. These booties permit the diver to walk comfortably over rocks, rocks with barnacles, and corals for entry and exit from the water when not wearing fins. Quick-release straps make donning and removal easy, which is especially important when the diver is making surf entries in the presence of waves and surges. Once the foot is inserted and the straps are tightened, the fin is not likely to be accidentally pulled off. If the diver dons fins before entering the water, the extended length of the blade makes walking through the water very challenging, especially if waves and surge are present and the diver is carrying a full complement of gear. In these circumstances, flat-bottomed, relatively rigid fins that are not too long provide definite advantages. Finally, the straps (as well as the back of the heel portion of the foot holder) are the most vulnerable to breakage. While removable straps can be replaced, a tear in a strap that is part of the body of the fin or the shoe-type foot portion will necessitate replacement of the fins. The incorporated (nonreplaceable) strap or partial-shoe foot holder of the fin also has advantages and disadvantages. It avoids the need to use a bootie, reduces bulkiness, eliminates the need to add extra weights to compensate for the neoprene in the bootie, and makes the fin more streamlined. Usually divers wear a sock or a thin neoprene liner to prevent chafing of the skin with this type of foot holder. The strap and/or partial-shoe foot holder is harder to insert the foot into and is more likely to be pulled off the foot in rough water conditions. This is because the tension of the straps cannot be adjusted to ensure the tightest, most comfortable fit. If the shoe type is used, the opening of the shoe has to be large enough to allow the foot to be inserted, often with some difficulty. However, once the foot is in the fin, it is not held as snugly as with a pocket and adjustable strap over the heel. We have found little information about the advantages of one swim fin type over another. One study showed that the preferred swim fin was the one that the subjects had the most previous experience with. Consequently, selection of fins, in contrast to much of the other diving equipment, tends to be a highly individual and experience-based decision. | Click graphic to enlarge image |
References1. Hong SK, Henderson J, Olszowka A, et al. Daily diving pattern of Korean and Japanese breath-hold divers (ama). Undersea Biomed Res 1991;18(5-6):433-443. 2. Rahn H. The physiological stresses of the Ama. In: Rahn H, ed. Physiology of breath-hold diving and the Ama of Japan. Washington, DC: National Academy of Sciences. National Research Council Publication 1341, 113. 3. Adrian MJ, Mohan S, Karpovich PV. Energy cost of leg kick, arm stroke, and whole crawl stroke. J Appl Physiol 1966;21(6):1763-1766. 4. Donald KW, Davidson WM. Oxygen uptake of "bottled" and "fin swimming" divers. J Appl Physiol 1954;7(1):31-37. 5. Andersen LK. Energy cost of swimming. Acta Chir Scand Suppl 1960;235:169. 6. Specht H, Goff LG, Brubach HF, Bartlett RG Jr. Work efficiency and respiratory response of trained underwater swimmers using a modified self-contained underwater breathing apparatus. J Appl Physiol 1957;10(3):376-382. 7. Lanphier E. Oxygen consumption in underwater swimming. Fed Proc 1954;13:84. SOURCE - Diving Science |
